On the TIntermNode class there is no Derived *getAsDerived() for
TIntermBranch. This is needed for the shader debugger to determine
the node type.

BUG=angleproject:1320

Change-Id: I9ce07017ccdb206c06c296b003b31eab6c65653c
Reviewed-on: https://chromium-review.googlesource.com/328281
Tryjob-Request: Tibor Ouden, den <tibordenouden@gmail.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>